1. Field of the Invention
The invention relates to a process and an apparatus for producing a seamless cellulose-based tubular film by extrusion of an aqueous cellulose-N-methylmorpholine N-oxide (NMMO) solution through an annular die into a spinning bath.
2. Description of the Related Art
Cellulose is insoluble in the customary solvents and has neither a melting point nor a melting range and can therefore not be melt-processed. For this reason, cellulose is usually chemically converted for producing tubular casing for foods, such as a sausage casings, these processes breaking down the cellulose, i.e. the average degree of polymerization of the cellulose is decreased. Processes of this type are technically very complex and are correspondingly expensive to operate.
The viscose process is currently preferred for the extrusion of cellulose-based tubular films. In this process, the cellulose is reacted with sodium hydroxide solution and then with carbon disulfide. This produces a cellulose xanthogenate solution which is extruded through a spinneret or annular die into a spinning bath or precipitation bath. The cellulose is regenerated using further precipitation and washing baths.
It has long been known that cellulose is soluble in oxides of tertiary amines. Currently, the most suitable solvent for cellulose is N-methylmorpholine N-oxide (NMMO). The cellulose dissolves therein without being modified chemically. No breakdown of cellulose chains takes place. The production of suitable spinning solutions has been disclosed (DD 218 104; DD 298 789; U.S. Pat. No. 4,145,532; U.S. Pat. No. 4,196,282; and U.S. Pat. No. 4,255,300).
Yarns may be produced from the solutions of NMMO by extrusion into a spinning bath (DE-A 44 09 609; U.S. Pat. No. 5,417,909). WO 95/07811 (=CA 2 149 218) also discloses a process for producing tubular cellulose films by the amine oxide process. This process features cooling the extruded film with a cooling gas immediately below the annular gap of the extrusion die. In accordance with EP-A 662 283, the extruded tubular film is cooled from the inside using liquid.
Recovery and purification of the NMMO is described in DD 274 435. Since the cellulose is not chemically modified in the process, less equipment is required. In the amine oxide process, no gaseous or aqueous waste products are produced, so that there are no problems with the exhaust air or waste water. Such processes are therefore becoming of increasing importance.
EP-A 0 686 712 describes the production of flexible cellulose fibres by the NMMO spinning process. In this process, a cellulose solution in water-containing NMMO is expressed through a spinneret, conducted through an air section into an NMMO-containing aqueous precipitation bath and then washed, after-treated, and dried.
According to WO 93/13670, a seamless tubular food casing is produced by extrusion of a solution of cellulose in NMMO/water using a special extrusion die. There is an air section between the extrusion die and precipitation bath. A feature of this process is a specially shaped hollow mandrel, through which the precipitation liquid can also circulate in the interior of the tube. In the air section, the interior of the extruded tube is virtually completely filled by a hollow mandrel and precipitation liquid. The tube is not stretched transversely in the course of this.
WO 95/35340 describes a process for producing blown cellulose films in which an underivatized cellulose which is dissolved in NMMO is used.
In the extrusion of a tubular film from an annular die into a spinning bath, the tubular film not being mechanically supported within this spinning bath, it is found to be disadvantageous that the accuracy of caliber of tubular film filled with an inner bath solution cannot be maintained, and irregular fluctuations in caliber diameter of the tubular film occur during the continuous extrusion process.